Reviewing DMTF OVF as a “preliminary standard”

OVF 1.0.0d is out as a “preliminary standard” so I gave it a quick read over the weekend. Things have not changed much since the “work in progress” document published this summer, which itself wasn’t a big change from the original specification. As I wrote in the review of the “work in progress”, the DMTF tightened the language of the specification more than it added features.

Since there aren’t too many technical changes (see the end of this post if you’re interested in a few), the interesting discussion is about the marketing of this specification. And boy does it have wings on that front. The level of visibility the specification has received is pretty amazing, especially considering that it doesn’t really do that much technically. But you wouldn’t know it by reading all the announcements about OVF:

An Open Source “implementation” of OVF has been created. I put “implementation” between quotes because since OVF per se doesn’t do much its implementation is mostly a specialized command line editor for its XML descriptor. It requires a a vendor-specific runtime for deployment/activation. This is not a criticism of the open source project BTW, just a statement of fact about the spec.

Microsoft support for OVF in products is supposedly “on the board” which doesn’t mean very much but their overall marketing/PR response to OVF has been surprisingly positive for a standard that they don’t control.

I have criticized the DMTF marketing efforts in the past (“give away pens and key chains”) but I must admit that, to the extent that DMTF had a significant role in promoting OVF adoption (in addition to marketing efforts directly from the vendors), it is a very nice marketing success. Well done, and so much for my cynicism. OVF may also have benefited from all the interest in the general topic of virtualization/cloud standards (the “cloud” association is silly, of course, but as we’ve just seen I am not a marketing genius) and the fact that there isn’t much else to talk about on these topics. So by default OVF becomes the name to put on your “standards” banner. Right place at the right time for the vendors behind it.

Speaking of the vendors, I have no insight into the functioning of the OVF working group, but judging by the specification’s foreword VMware is throwing plenty of resources at DMTF: it employs the working group chair and both co-editors, which is pretty atypical in my experience in standards efforts. People are usually sensitive to appearances of one company having disproportionate influence and try to distribute responsibilities around, at least on paper. Add to this VMWare’s recent ramp-up at the DMTF board level. They seem to know what they want. And indeed I can see how the industry leader would want some basic level of standardization, but not too much, which is currently just what OVF offers. We’ll see what’s next in store, if anything.

The specification itself is not marketing-free. According to line 122, “it supports the full range of virtual hard disk formats used for hypervisors today, and it is extensible, which will allow it to accommodate formats that may arise in the future”. Sure, in the same way that my car fully supports passengers of all nationalities (and is extensible enough to transport citizens of yet-to-be created countries – and maybe even other planets, as long as they come with buttocks to sit on). Since OVF doesn’t really do anything with the virtual hard disk formats, it can “support” pretty much any such format.

Speaking of extensibility, OVF clearly tries to have a good story there. Section 7.3 tries to move away from the usual “hey, it’s XML, you can add elements/attributes anywhere” approach towards the definition of new “sections”. This seems a bit drastic. Time will tell if this is visionary or short-sighted. OVF also plans to move towards “an extension model based on the design of the open content model in XML Schema 1.1”. I am not following XSD 1.1 too closely, but it is wise for OVF to not build too much dependency on it at least for now. And it seems to me that an extension model is not something that you plan to “plan […] to add” but rather something you need to define from the start (sounds like the good old “the next version will add versioning support”, or “no keyboard detected, press F8 to continue”).

But after all this comes what looks to me, from an extensibility perspective, like a big no-no: using (section 8.1) simple strings (e.g. “vmx-4”, “xen-3”) to represent types of virtual systems. You’d think that in 2008 people would have heard about URIs as a way to allow extensibility and prevent name clashes. On further reading, this doesn’t seem to be the fault of OVF as they get this property (vssd:VirtualSystemType) straight out of the politely named DMTF SVP (System Virtualization Profile) specification, itself a preliminary standard. But that’s not much of an excuse because I suspect large overlap of participation between the two groups and in any case you don’t have to take dependencies on something that’s not right (speaking as someone who authored several specs that took a dependency on WS-Addressing, I shouldn’t give lessons). In any case, I am not on top of all virtualization-related work in DMTF but it seems to me that if they are not going to use URIs then someone should step up and maintain a registry of these identifying “virtual system type” strings.

BTW, when left to its own device OVF does a better job. For example, it properly uses URIs to identify the virtual disk format (section 5.2).

One of the few new features is the addition of the ovf:bound attribute on virtual hardware element items (section 8.3) to specify whether the item description represents the normal, minimal or maximal allocation. My heads spins a bit when trying to apply this metadata to the rasd:Limit property (with ovf:bound=”min” the value of the rasd:Limit element would represent the minimal value of the maximum quantity or resources that will be granted, which takes some parsing effort), but I think it more or less squares out.

The final standard should not differ greatly from this version, so at this point we pretty much know what OVF will be technically. The real question is how it will be used and what, if anything, is going to come to complement it.